Microporous membranes have many important applications in the electronics industry, food processing industry, the processing of biological materials, as sterilizing filters and the like. To make a microporous membrane with desired characteristics, known methods typically require that the temperature of the reaction conditions be carefully monitored and controlled within a narrow range. Normally as the temperature of a casting solution increases so does the average diameter of the pores in the resulting membrane, all other variables being constant. If the temperature of a casting solution is too high or low, the resulting membrane can have undesirable characteristics. The equipment, manpower and energy needed to control and monitor the temperature of reaction conditions makes known methods for making microporous membranes impractical and expensive.
It would be desirable to develop a method for making a microporous membrane that enables one skilled in the art to control the characteristics of the membrane without having to maintain careful control of temperature.
In membrane science, the term "skinned" usually refers to a membrane structure where the surface pores are much smaller than the interior pores. The skin is quite thin, usually on the order of 5% or less of the total membrane thickness and the transition to the much larger interior pores is abrupt. Skinned membranes have a disadvantage in that careful orientation to the fluid flow must be rigorously maintained otherwise throughput is drastically reduced. The flow through skinned membranes is generally controlled by the size of the pores in the skin layer and thus independent of the thickness. Also maintenance of precise membrane parameters can be difficult in that any imperfections in the skin layer will result in loss of critical separation capability. On the other hand, the term "asymmetric" refers to a condition where the surface pores, although somewhat smaller than those of the interior, nevertheless are similar in size. The pores enlarge gradually from the surface through the interior structure.
U.S. Pat. Nos. 4,629,563 and 4,774,039 disclose manufacturing "highly asymmetric" polyethersulfone membranes. The patents teach that the highly asymmetric and skinned structures are produced only from metastable mixes which precipitate polymer by any non-solvent addition, or by cooling the mix, or even upon standing. It is significant that later patents U.S. Pat. Nos. 5,171,445 and 5,188,734 from the same assignee modify the procedure and teach reducing air exposure to as little as possible (in any case less than 0.5 second). This is done to improve the physical properties of the product. Surprisingly, even when the more open side of the membrane faces the fluid stream there is reduced throughput compared to less asymmetric membranes.
U.S. Pat. No. 4,976,859 discloses a polyethersulfone membrane with an unusual cell structure. This product must be highly skinned because it shows " . . . a flow of fluid . . . which is largely independent of membrane thickness."
U.S. Pat. Nos. 4,900,449, 4,964,990, and 5,108,607 describe preparing hydrophilic polyethersulfone membranes by forming a solution of a hydrophobic polymer starting material and adding a high molecular weight (up to 10,000 daltons) polyethylene glycol prior to casting the polymer into a membrane. The high molecular weight polyethylene glycol is responsible for the initial hydrophilicity of the resulting polyethersulfone membrane. However, under process conditions the high molecular weight polyethylene glycol, a known wetting agent, slowly leaches out which both contaminates the filtrate and causes reduced wettability with time.
U.S. Pat. No. 5,444,097, induces phase separation in a polyethersulfone solution by heating.
It is an object of this invention to produce essentially unskinned and symmetrical polyethersuffone membranes from stable polymer mixes, which membranes have flow rate and throughput properties at least as good as prior art membranes in a range of pore sizes and with easily controlled process parameters. It is another object to produce a hydrophilic polyether sulfone membrane having very low extractables.